<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>undefined</title>
</head>
<body>
<p>变量的值如果不存在则该变量值为<b>undefined</b></p>
<p id="demo"></p>
<script>
    var person;
    document.getElementById("demo").innerHTML =
        person + "<br>" + typeof person;
</script>
<!--null 和 undefined 的值相等，但类型不等-->
</body>
</html>